Creating generic Objective C relationship model - objective-c

Question

I'm trying to setup generic model for retrieving data from a backend and processing the data in my iPhone app. I'm trying to accomplish this by creating a base-class for all class in which all logic is defined for retrieving, saving and linking data. This last part is the part I'm struggling on.
In every class I define the object name (in Core Data) and if a certain object has relationships to other objects, also and array of the names of the objects with two arrays (one for hasOne and one for hasMany).
However I can't seem to accomplish cast retrieved objects to a certain class which is stored as a string. This is the code I currently have:
/**
Update the relationsships of an object which has relationsships from the type 'HasOne'.
#param object
Object from which the relations has to be updated.
*/
- (void)updateRelationshipsHasOne:(id)object {
for(NSString *relation in self.hasOne) {
Class class = NSClassFromString(self.table);
id anInstance = [class alloc];
anInstance = object;
NSString *key = [[NSString alloc] initWithFormat:#"%#_id",[relation lowercaseString]];
id delegata = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *context = [delegata managedObjectContext];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:relation inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat: #"db_id == %i",[[object valueForKey:key] intValue]];
[fetchRequest setPredicate:predicate];
NSError *error;
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
if(fetchedObjects && [fetchedObjects count] > 0) {
id otherInstance = [fetchedObjects objectAtIndex:0];
SEL s = NSSelectorFromString([[NSString alloc] initWithFormat:#"set%#",relation]);
[anInstance performSelector:s withObject:otherInstance];
}
}
}
If I cast both 'anInstance' and 'otherInstance' to the desired classes, everything goes well. However the whole purpose is to make the method generic, so I need some way to be able to cast both variables to Classes which are stored as strings.
Thanks a lot in advance for any help!
Edit:
The error message I'm receiving is:
2016-03-21 20:34:19.024 DeDamen[12303:834645] -[Heat setEvent]: unrecognized selector sent to instance 0x7fe2829d8cd0
2016-03-21 20:34:19.026 DeDamen[12303:834645] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Heat setEvent]: unrecognized selector sent to instance 0x7fe2829d8cd0'
*** First throw call stack:
(
0 CoreFoundation 0x0000000109e68e65 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x00000001094c0deb objc_exception_throw + 48
2 CoreFoundation 0x0000000109e7148d -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
3 CoreFoundation 0x0000000109dbe90a ___forwarding___ + 970
4 CoreFoundation 0x0000000109dbe4b8 _CF_forwarding_prep_0 + 120
5 ******* 0x0000000105d255ab -[CommonBase updateRelationshipsHasOne:] + 1627
6 ******* 0x0000000105d24e60 -[CommonBase updateRelationships:] + 256
7 ******* 0x0000000105d248a9 -[CommonBase save:handler:] + 1065
****
)
libc++abi.dylib: terminating with uncaught exception of type NSException
In here the model 'Heat' refers to the object 'anInstance' and the model 'Event' refers to the object 'otherInstance'. The relations ship is: a Heat has one Event and an Event has many Heats'